Loop antenna input circuits



March 18, 1952 w. F. SANDS LOOP ANTENNA INPUT CIRCUITS 2 SHEETS-SHEET 1 Filed Oct. 6, 1948 INVENTOR WILLIAM}? SANDS ATTORNEY March 18, 1952 NDS 2,589,736

LOOP ANTENNA INPUT CIRCUITS Filed Oct. 6, 1948 r 2 SHEETS-SHEET 2 fan 600 I 0 /60 A7 0 Q 6 r is Z I \Q X w & M 7 Q 2'0 40a 60a 50a 0aa flea /4aa /aa Ava I V peipz/i/vcy wnzoq/c'zis INVENTOR WILLIAM 1". SAND:

ATTORNEY Patented Mar. 18, 1952 LOOP ANTENNA INPUT CIRCUITS William F. sa ds, Haddo Radio Corporation of of Delaware nfield, N. J assignor to America, a corporation Application October 6, 1948, Serial No. 53,078

7 Claims.

This invention relates generally to antenna input circuits and particularly to permeabilitytuned loop antenna circuits for radio receivers.

A loop antenna for a broadcast receiver is conventionally associated with a variable capacitortuned input circuit. However, a capacitor-tuned input circuit used in connection with a loop antenna has a number of disadvantages. Thus, in order to obtain the desired tuning range, the variable capacitormust have a large capacitance at the low frequency end of the tuning range, with the result that the total impedance of the tuned circuit at that frequency is low. This, in turn, reduces the sensitivity of the antenna input circuit at the low frequency end of the tuning range, and the signal-to-noise ratiois poor. Another disadvantage of a capacitor-tuned antenna input circuit is theunpleasant acoustic feedback howl which may be caused by vibration of the capacitor plates.

These disadvantages of prior antenna input circuits can be overcome if the loop antenna is associated with a permeability-tuned input circuit. A permeability-tuned circuit has the advantage of compactness and light weight, and furthermore, the circuit can be mounted in any part of the receiver chassis. Excellent performance has been secured in the past with a rod type antenna in conjunction with a permeabilitytuned input circuit. However, a rod type antenna is unsuitable for broadcast receivers in view of its detrimental body capacity effects, its low sensitivity in shielded buildings and the unsightly appearance of a rod extending beyond the cabinet. For these reasons a loop antenna is the preferred antenna in the broadcast receiver field. Unfortunately, the sensitivity of a loop antenna coupled to a conventional permeabilitytuned input circuit is very low. Even the best previously known permeability-tuned loop input circuits have sensitivities that are low compared to that of a variable-capacitor tuned input circuit. The sensitivity of a conventional permeability-tuned loop input circuit is low because-the resonant circuit coupled to the loop antenna must have a comparatively large inductance so that the broadcast range can be covered by variation of the effective inductance of the tuned circuit. Accordingly, the inductance of the loop antenna must be made small which results in a reduced pickup of the electromagnetic wave energy and thus explains thelow sensitivity. One of the best available prior circuits of this type'has been disclosed by G. L. Beers, in U. S. Patent 2,383,286. Even this circuit has a low sensitivityatthehigh frequency end of the broadcast band and its performance is therefore not optimum for use in broadcast receivers.

It is the principal object of the present invention, therefore, to provide novel permeabilitytuned loop antenna circuits which have sensitivities comparable to or better than that of a variable capacitor-tuned antenna input circuit.

A further object of the invention is to provide more efiicient coupling between a loop antenna and the permeability-tuned input circuit of a radio-receiver with a resultant improvement in sensitivity over the entire tuning range and particularly over the high frequency end of the tuning range.

A further object of the invention is to provide permeability-tuned loop antenna circuits having a high signal-to-noise ratio and a high Q and hence a better performance.

A loop antenna input circuit in accordance with the present invention may be termed a duo auto-transformer, that is, the loop antenna is tapped at an intermediate point as is the inductor associated with the loop. Thus, the loop antenna as well as the inductor functions as an auto-transformer. A circuit of this type combines the advantages of a series tuned circuit associated with a loop antenna with that of an auto-transformer where an intermediate point of the inductor is connected to a loop antenna terminal as taught by Beers. Accordingly, the loop antenna circuit of the invention combines the advantages of the series tuned circuit havin a high sensitivity at the high frequency end With the advantages of the Beers circuit which has a high sensitivity at the low frequency end of the tuning range.

The duo auto-transformer of the invention may also be modified to form an inverted duo auto-transformer by opening an intermediate portion of the loop antenna to provide a pair of terminals. These circuits may be further improved by the provision of an additional capacitor which will further increase the sensitivity at the high frequency end of the tuning range.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

Fig. 1 is a circuit diagram of a portion of a twoantenna portions. terminal a is'connected to an'intermediate point-1 iof-lodp antenna I.

through lead 20 on control grid 16 through resistor 2| having its cathode end grounded for alternating currents through capacitor 22. Radio-frequency output circuit 23 is connected between a suitable voltage source indicated at +13 and the plate of amplifier 11. Output circuit 23 includes inductor 24 and capacitor 25 connected, in parallel. Inductor 24 includes paramagnetic core 26 which may be moved to vary the inductance of inductor 2-1 and thereby the resonant frequency of output circuit 23.

Fig. '7 is a circuit diagram of another embodi- .ment of a loop antenna circuit in accordance with the invention; and

Figs.8 and 9 are graphs which represent the sensitivity of the circuits of Figs. 1 to 6 over the broadcast tuning range.

I Referring now to the drawings in which like components are designated by the same reference numerals, and particularly to Fig. 1 there is "illustrated an antenna input circuit in accord- I ance with the present invention including loop antenna I. Loop antenna l consists of two portions, 2 and 3, which are connected together .to form one continuous loop. Loop antenna I has: three-terminals, 4 and 5, being connected to the free ends of antenna portions 2, 3 and terminal 6 being connected to the junction pointof the In other words, antenna Loop-antenna lisconnected to permeabilitytunedinput circuit 1- comprising inductor 8 which includes core ii! of paramagnetic material. A

paramagnetic material is defined as amaterial having a magnetic permeability greater than that -of a vacuum, which is unity. The magnetic permeability or a paramagnetic material maybe independent of the magnetizing force or it may vary with the magnetizing force,--in which case the material is called ferromagnetic. Antenna iii terminal 5- is' connected to a point intermediate 0f the ends of inductor 8 so that inductor 8 consists-of two coil portions, l l and I2. Antenna terminal 6 is connected to the low alternating-current pot'ential terminal of inductor 8. Antenna terminal't is connected to ground, that is,-to a point of fixed alternating-current reference po- ,:tential. It-is preferred to ground antenna terminal i to the common return lead or chassis, shown, because if antenna terminal 4 is not directly grounded but is maintained at some 'other relatively fixed alternating current potential there may be anefi'ectiveseries resistance between antenna terminal 4 and ground which will reduce the efiective Q of the input circuit.

However, in some cases it may be necessary or desirable to maintain antenna terminal 4 at a fixed. alternating-current potential instead of grounding it directly.

Capacitor M is effectively connected across -inductor B and loop antenna portion 2. Thus, capacitor It is preferably connected, as .illustrat'ed, between the highalternating-current potential terminal of inductor 8 and ground. The loop antenna input circuitof the invention generally 'indicatedby box and which maybe termed a duo auto-transformer circuit, is coupled ;to control grid IG of radio frequency amplifier "l1 throughcoupling capacitor IS. The cathode" of-amplifier l! is grounded as illustrated andits sc're'en-grid is connected to +3 through a suitable .Ldropping' resistor which may be bypassed to ground as illustrated. An AVC voltage maybe derived in a'conventional manner and impressed Output circuit 23 is coupled to signal grid 28 of pentagrid converter tube 30 through coupling capacitor SE. The AVC voltage is impressed on signal grid 28 through lead 2% and resistor 3'2. The screen grids of converter tube are connected to +3 as illustrated.

The oscillator section of converter tube 35} includes oscillator grid 33 and cathode 3 5. Oscillatory circuit 35 is coupled between grid 33 and cathode 34 and includes inductor 35 across which are connected capacitors 3'! and 33 arranged in series. The junction point of capacitors 3?, 33

is connected to cathode 34 which is grounded through choke coil 1%. The high alternatingpotentialterminal of oscillatory circuit 35 is cou- 'pled to controlgrid '33 through coupling ca- :pacitor 4!. between oscillator. grid 33 and ground. Inductor Grid leak resistor 42 is connected 36 includes paramagnetic core d3. Oscillatory circuit 3'5'is connected in the manner of a Colpitts oscillator. The intermediate frequency "wave developed by frequency converter 30 may ibeiobtained from resonant circuit 44 coupled to Set the anode of converter 30 to which is coupled outputcircuit. 45.

Paramagnetic cores I0, 25 and es are preferably-movable in unison as indicated at 46. The permeability of paramagneticcore [0 should be substantially higher than that of, paramagnetic core 43. Thus, it is preferredthat the efiective permeability of inductor 8 be larger than 20 and a of the .order of 30. Theeffective permeability :of :an inductor maybe defined asthe ratio of the inductance of the inductor with its core fully inserted, divided by the inductance of the inductor withthe corefully removed. The effective permeability of an inductor is a function of -the geometry of. its coil andits core and. of the material. of' the. core. High effective permeabili- .ties may'readily be obtained with a paramagnetic core'consisting of a ferrite which consists metals with ferric oxide F6203.

'formulalof a ferrite is M810: MezO: F6203, where of a mixture of'the .oxidesiof various bivalent The general Mermay be aimetal'such-as nickel, copper, man- :ganese and magnesium. and where Mez maybe a metal such-as zinc or cadmium. A ferrite core .m'ay'be .produced by intimately mixing the finely divided metallic oxides, compacting the mixture by compression molding and heating it to a temperature of between 1,000 and 1,600 degrees centigrade.

Onthe otherhand, paramagnetic core 23 may (consist of one ofthe'various types of powdered iron and a suitable casting resin so, thatthe perrmeability o'f'core .63 is considerably lower than "that of core lfl. The permeability of paramagnetic core 26' should be .not larger than that of core. I0 and not smaller than that of core 43.. In

"other words cor'e :26 may consist-of a'ferrite ,having a high permeability or of powdered iron having alow. permeability. ,It is also. feasibleto make the permeability of core .26 smaller than that voffcore'l'lil'while that .of core 43 is, still high frequency end. At

smaller than that of core 26. In that case, both cores 26 and 43 may consist of powdered iron but core 43 may contain a larger amount of binder to reduce its permeability.

It is, of course, essential that input circuit I should have the highest possible effective permeability in order to tune the loop input circuit over the required tuning range and to obtain good sensitivity. The effective permeability of radio frequency circuit 23 should also be high but need not necessarily be as high as that of input circuit 1 since the tuning range is not limited by a fixed loop inductance. On the other hand, the effective permeability of oscillatory circuit must be somewhat lower than that of input circuit I because the tuning range of the oscillatory circuit is considerably smaller than that of the input circuit. The above mentioned materials are admirably suited for this purpose.

The operation of the loop antenna input circuit of the invention may best be understood by reference to Fig. 2 which is the equivalent circuit diagram of the duo auto-transformer circuit. For convenience, antenna portions 2 and 3 have been shown separately. It is to be understood that loop antenna I may also be arranged physically with two distinct portions as illustrated schematically in Fig. 2. It will readily be seen that antenna portion 2 is connected in series with inductor 8 between ground and the high alternating-current potential terminal of the circuit. The two loop portions 2 and 3 are physically arranged in such a manner that their inductances are in aiding phase. Furthermore, the capacitive coupling of the two loop portions 2 and 3 is also 5.

in aiding phase Fig. 1.

The loop antenna input circuit of Fig. 3 combines the advantages of a conventional loop antenna connected to a series tuned circuit (corre sponding to Fig. 2 with loop portion 2 removed) with that of a loop antenna coupled to an autotransformer (corresponding to Fig. 2 with loop portion 2 short-circuited) as taught by Beers.

The conventional loop antenna connected to a series tuned circuit is most sensitive at the high frequency end of the tuning range. This can be readily understood by the following analysis. When the core is fully inserted into the series coil of the circuit, the circuit is tuned to the low frequency end of the band and most of the inductance of the antenna input circuit is represented by the series coil and its core. Accordingly, the loop antenna inductance is a comparatively small portion of the entire inductance of the input circuit resulting in a reduced efficiency of the circuit because less voltage is delivered to control grid I0. At the high frequency end of the tuning range the core is withdrawn from the series coil and accordingly the inductance of the series coil is a smaller portion of the entire inductance of the loop antenna input circuit. Since now the inductance of the loop antenna represents a larger portion of the total inductance, the efficiency of the antenna circuit is improved at the high frequency end of the tuning range.

The Beers loop antenna circuit which includes an auto-transformer, is more sensitive at the low frequency end of the tuning range than at the the low frequency end of the tuning range the core fully penetrates the auto-transformer. Accordingly, the loop antenna is effectively tapped across a large portion of the inductance of the coil. The circuitaccordingly as may be seen by reference to is very eflicient and has a good sensitivity. However, at the high frequency end of the tuning range the core is withdrawn from the auto-transformer. Consequently, the loop antenna is effectively tapped across a small impedance because the tap of the auto-transformer may be considered to move effectively to such a position that the antenna is connected across a smaller portion of the coil. It will be evident that this impedance mismatch results in a loss of sensitivity at the high frequency end of the tuning range.

The duo auto-transformer circuit of the invention as illustrated in Figs. 1 and 2 combines the advantages of the two conventional circuits and therefore has a high sensitivity across the entire tuning range which is comparable to or larger than that of the conventional variable capacitor tuned antenna input circuit. Thus, at the low frequency end of the tuning range core II) fully penetrates inductor 8. At this position of core Ill the circuit has the high sensitivity of the Beer's circuit which is still further increased due to the presence of antenna portion 2 serially connected with respect to inductor 8. At the high frequency end of the tuning range, core I0 is fully withdrawn from inductor 8. The core should be withdrawn in the direction indicated by the arrows Figs. 1 and 2, that is, toward coil portion II. Now, the comparatively smaller sensitivity of the Beers circuit is increased by loop portion 2 in the manner pointed out hereinabove.

Loop antenna I preferably consists of a Litz wire, that is, a stranded cable where the strands are mutually insulated from each other. This will increase the Q of the circuit and thereby improve its performance. For the same reason, inductor 8 should consist of a Litz wire. Furthermore, inductor 8 should preferably be wound with a universal progressive winding, that is, with a multilayer winding which will increase the Q of the coil and permit a higher ratio of inductance to capacitance of tuned circuit 1. This, in turn, will also improve the sensitivity of the tuned circuit. Furthermore, antenna portion 2 should include at least 30 per cent of the total number of turns of loop antenna I, but may include up to 50 per cent of the total number of antenna turns. This will provide a higher antenna inductance in series with inductor 8 which contributes to the better performance of the circuit at the high frequency end of the tuning range.

In one particular embodiment of the circuit of Fig. 1, loop antenna I had a rectangular shape with rounded corners and was wound in flat form with 28 turns. Antenna portion 2 had 12 turns and antenna portion 3 had 16 turns; the mean area of the loop was 0.022 m. Inductor 8 had a universal progressive winding of Litz wire and a coil length of 1 inches, the coil being wound on a 0.271 inch outer diameter thin-walled coil form. Core I0 had an outer diameter of 4 inch and a length of more than 1 /2 inches and consisted of ferrite.

Fig. 8 illustrates the relative field strength in microvolts per meter which is re uired at 10010 antenna I to produce an AVC voltage of 1 volt, these voltages being plotted against fre uency in kilocycles. Curve 50 illustrates the performance of the loop antenna circuit of Figs. 1 and 2. It will be noted that the sensitivity increases as curve 50 approaches the bottom of the graph. Curve 50 shows clearly the superior sensitivity of the loop antenna input circuit of Figs. 1. and 2 particularly at the high frequency end of the tuning range.

one of its terminals 85 connected to the low alterhating-current potential terminal of inductor 8; The other terminal 86 of conductor 82 is connected to another intermediate, point of inductor 8. It will be seen that terminal 84 is connected to a point on inductor 8, intermediate tap 86 and terminal 85.

Antenna conductor 82 is connected in the manner of the Beers circuit referred to. Accordingly, the circuit performs better at the low frequency end of the tuning range than at the high frequency end. This is due to the fact that conductor 82 is connected across a small impedance at thehigh frequency end of the tuning range as has already been explained. Antenna conductor 8| adds to the sensitivity of the circuit and functions essentially like another Beers circuit. At the high frequency end of the tuning range, that is,'when cor'e' 'lfl'is removed from the inductor, there is an inductance represented by conductor 8| in series with a portion of inductor 8. Curve 88 of Fig. 8 illustrates the' sensitivity of the circuit of Fig. 7. The sensitivity may be improved by connecting antenna terminal 85 to ground either directly or through a capacitor thereby to maintain the alternating-current potential of the terminal constant.

There have thus been described various permeability-tuned loop antenna input circuits which have a sensitivity which is comparable to or greater than that of a variable capacitor tuned loop antenna input circuit. The circuits of the present invention are superior to prior art permeability-tuned loop antenna circuits and haveall the advantages inherent in permeability tuners.

What is claimed is:

1. In a radio receiver, a loop antenna having at least three terminals, at least one of said terminals being connected to an intermediate point of said antenna, an inductor including a paramagnetic core, means for moving said core to vary the inductance of said inductor, one of said antenna terminals being maintained at a relatively fixed alternating-current potential, the other two antenna terminals being connected respectively to one terminal and to an intermediate point of said inductor, a first capacitor efiectively connected between the free terminal of said inductor and said one of said antenna terminals, and a second capacitor connected between said free terminal and said intermediate point of said ind ictor.

2. In a radio receiver, a loop antenna having a first and a second terminal, an inductor including a paramagnetic core, means for moving said core to vary the inductance of said inductor, said first antenna terminal being maintained at a relatively fixed alternating-current potential, said second antenna terminal being connected to an intermediate point of said inductor, an intermediate point of said antenna being connected to one terminal of said inductor, a first capacitor efiectively connected between the other terminal of said inductor and said first antenna terminal, and a second capacitor connected between said other terminal and said intermediate point of said inductor.

3. In a radio receiver, a loop antenna having at least three terminals, at least one of said terminals being connected to an intermediate point of said antenna, an inductor including a coil and a paramagnetic core, means for moving said core to vary the inductance of said inductor, said core consisting of two portions, one of said portions 10 having a higher permeability than the other portion, said portions having both a low loss and being of such a length that regardless of the movement of said core said core is always fully within said coil, thereby to maintain the Q of said inductor at a substantially constant value, one of said antenna terminals being maintained at a relatively fixed alternating-current potential, the other two antenna terminals being connected respectively to one terminal and to an intermediate point of said inductor, a first capacitor effectively connected between the free terminal of said inductor and said one of said antenna terminals, and a second capacitor connected between said free terminal and said intermediate point of said inductor.

l. In a radio receiver, a loop antenna having itsfree ends connected together to form a first terminal, said antenna having an intermediate portion opened to provide a second and a third terminal, an inductor including a paramagnetic core, means for moving said core to vary the inductance of said inductor, said first antenna terminal being connected to a terminal of said inductor, said second antenna terminal being maintained at a relatively fixed alternating-current potential, said third antenna terminal being connected to an intermediate point of said in ductor, a first capacitor connected between the free terminal of said inductor and said second antenna terminal, and a second capacitor connected between said free terminal and said intermediate point of said inductor.

5. In a radio receiver, a loop antenna having its free ends connected together to form a first terminal, said antenna having an intermediate portion opened to provide a second and a third terminal, an inductor including a paramagnetic core, means for moving said core to vary the inductance of said inductor, said first antenna terminal being connected to a terminal of said inductor, said second antenna terminal being maintained at a relatively fixed alternating-current potential, said third antenna terminal being connected to an intermediate point of said inductor, a first capacitor connected between the free terminal of said inductor and said second antenna terminal, and a second capacitor connected between said intermediate point of said inductor and said second antenna terminal.

6. In a superheterodyne receiver, a loop antenna having a first and a second terminal, said first antenna terminal being maintained at a fixedpotential, a first inductor including a first paramagnetic core, said second antenna terminal being connected to an intermediate point of said inductor, one terminal of said inductor being connected to an intermediate point of said antenna, a capacitor connected between said first antenna terminal and the free terminal of said inductor, a radio frequency amplifier stage having its input terminals connected to said capacitor, a radio frequency output circuit coupled to said amplifier stage and comprising a second inductor, said second inductor including a second paramagnetic core, a local oscillator comprising an oscillatory circuit, said oscillatory circuit having a third inductor including a third paramagnetic core, and means for moving said cores in unison to vary simultaneously the inductance of said inductors, the permeability of said first core being larger than that of said third core and the permeability of said second core being not larger than that of said first core and not smaller than that of said third core.

7. In a supe 'hete r odyne reoeiver, a loop antennahaving at least; three j:ermi na1s,' a'-t least one 015 said terminals beingconneoted'to an inter mediate-point of said antenna; gfi si; i-nduojaor including a first paramagnetic core; one" ofsaid antennaterminals being maintained-at afrelatively fixed alt rnating currenii potenpial, the other we antenna terminals being connected re siaectively to one terminal and to an intermediate point of said firsti-nduc tor, a first oapacijtoneffee tiile'ly connected between the fiee terminal oi said inductor and said one of said antenna terminals, a second oapacitor oonneoted-between said gee tefminal and saidintermediate point of said di iotona radio frequency amplifier stage haying its inplit terminals Connectedtosaid first oapaoiigor, a radio frequenoy output circuit coupled to Said am p lifief'stage and oomprising a second indlictdrfsaid second inductor inc1uding a eo9nd pai'ama'gnetic' dore; a local osoillator oompi'ising' an osci'llafio'i'y ci'i'u'itj'sa id os oillatofyciroliifi l av ing a third 'inuuetorincludinglafiiix d payamagnietioioore, andmeans for moving said co esfi' n joore einglar er, 'ohanihat or; Said 'thiid" un q T190. 1 m simu taneously the i du tanc fi ai indu s;ii em meability"o .s ai ifi'rist not larger than that of said first cote mino:

smaller than, that of 'said"third"core;

7 REFERENCES CITED The f ol1o wing fere eesj ate of I gecord in iphe J IQ th fiu t UNITED STATES PATENTS N mhfil. Nam Dat 25 3 Junie 3119 2,246,385 h i I June 17,1941 2250329 Fo'ste1 1 July 29519511 2,267;047 'Schaperj; "Dec." 23, 11941 fizz-4.2491 Ram I W; Aug; 21,1945

1 .383286 Beers '2,'422,381 'June 17,"1947 

